Not Applicable.
1. Field of the Invention
The invention relates to antenna configurations in a sectorized radio system coverage area served by a central hub station that employs frequency re-use of a given channel in different sectors.
2. Discussion of Related Art
Fixed point-to-multipoint radio systems operate at millimeter-wave frequencies or microwave frequencies. Such systems that operate at 24-42 GHz are sometimes termed Local Multipoint Distribution Systems (LMDS) or Local Multipoint Communications Systems (LMCS). These systems could be either one-way broadcast type, or two-way systems. Such systems that operate at approximately 2.5 GHz are sometimes termed Multi-channel Multipoint Distribution Systems (MMDS). Traditional cellular radio systems that use antennas provide sectorization of a cell.
In all cellular-structured radio systems, there is frequency re-use. In VHF and UHF bands, this means the re-use of a given channel in a nearby cell. At higher frequencies, such as millimeter-wave bands from 24-42 GHz, and possibly at lower microwave frequencies such as 2.5 GHz, radio propagation is closer to straight-line and base station or xe2x80x9chubxe2x80x9d antenna patterns are sharply defined. This opens the possibility of re-using a given channel within the same cell site in another sector.
An area to be served by a single hub is angularly-divided into k sub-sectors of equal angular arc, the sub-sectors radiating out from the hub. All antennas have identical sectoral radiation patterns approximately j sub-sectors wide. A first such antenna provides coverage of the first j sub-sectors using a first channel set. After a gap of the next i sub-sectors, the next j sub-sectors are covered by another antenna re-using the first channel set. This continues around 360 degrees of arc. The quantities k, i, j and k/(i+j) are integers. k/(i+j) greater than =2 and is equal to the number of times the first channel set is reused in the cell.
A second channel set is used to provide additional beams. These are similarly spaced i+j sub-sectors apart, but the first such beam is angularly staggered from the beams using the first channel set by (i+j)/n sub-sectors, where n is the total number of channel sets available. The quantity (i+j)/n need not necessarily be an integer. There are also k/(i+j) re-uses of the second channel set. The other channel sets, up to a total of n channel sets, are similarly staggered. In total, there are kn/(i+j) antenna beams using n channel sets. For complete coverage of 360 degrees with no gaps, n greater than 1+i/j is required.
The traffic capacity of a cellular radio system is proportional to the number of channel sets in use. A normalized measure of capacity is therefore the number of beams per hub divided by the number of channel sets available to the system. By this measure, a conventional cellular radio system with an omnidirectional antenna radiating n channel sets would have a capacity of 1.
The capacity of conventional cellular radio systems may be increased with narrow-beam sector antennas at the hub. However, adding antennas to an existing hub requires replacement or re-orientation of the existing antennas. Also, existing hubs have no appreciable redundancy so that equipment failure results in loss of service until repairs are complete. Still, redundancy may be made available but it requires doubling the number of radios.
One aspect of the present invention relates to an improved system of frequency reuse in millimeter-wave point-to-multipoint LMDS radio systems for broadband wireless access in which each channel can be reused several times within a single base station or hub in order to increase traffic capacity.
The cell area is divided into an integer number k sub-sectors of equal angular arc 360/k degrees each. Each antenna beam covers an integer number of j sub-sectors. Each channel set, out of a total integer of n sets, is re-used. The angular gap between edges of beams, which use the same channel set, is an integer number of i sub-sectors. The quantity k/(i+j), which is equal to the number of uses of each channel set, must be an integer greater or equal than 2. Each channel set is deployed with the same re-use pattern, but the channel sets are angularly staggered from each other by (i+j)/n sub-sectors, which is not necessarily an integer. To cover each part of the cell with a uniform number of channel sets using the minimum number of channel sets, then n=i+j, the angular stagger between antennas is one sub-sector, and there are k antennas. The number of channels within each channel set need not be equal. In total, the hub uses kn/(i+j) antenna beams, and the normalized traffic capacity is k/(i+j).
The invention may serve unanticipated locally higher capacity requirements in some portions of the cell by permitting an increase in capacity to be effected without the need to employ narrow-beam sector antennas at the hub. Additional antennas can be added to an existing hub without the need to replace or re-orient the existing antennas. Overlapping sector beams provide redundancy to increase reliability in the event of equipment failure.